Photoactive Yellow Protein (PYP) is a small cytosolic photoreceptor that is thought to be involved in the negative phototaxis of the bacterium Ectothiorhodospira halophilia. Its small size and the fact that it is not a membrane protein make it an ideal experimental system for establishing a structural paradigm for photon - protein interactions. In pursuit of this general goal, our lab is also involved in a study of PYP that uses time-resolved Laue diffraction experiments to study structural intermediates its photocycle.